Markdown streaming: every chunk re-scans the whole document, so a long response is still superlinear to stream

`parseMarkdownIncremental` now keeps its settled blocks at every document
length — after [#5378](https://github.com/facebook/astryx/issues/5378) the worst
chunk of a streamed document rebuilds 3 blocks whether the document is 50
paragraphs or 500. Streaming is still superlinear anyway, because four things in
the function touch the **whole input** on every chunk, cache hit or not:

| `parser.ts` | per chunk |
| --- | --- |
| `extractLinkDefinitions(input)` `:1958` | scans the whole document for `[ref]:` lines |
| `input.split('\n')` `:1970` | allocates an array of every line |
| `findSettledBoundary(lines)` `:1971` | walks every line, tracking fences from the top |
| `mergeSettledBlocks(...)` `:2033` | copies the whole block array |

Measured with the perf suite's `generateAIResponse` fixture, 50-char chunks,
best of 5, on `main` at `1b0fab89661`:

| paragraphs | chars | whole stream | cost of the last chunk |
| --- | --- | --- | --- |
| 50 | 6.6k | 3.3ms | 0.033ms |
| 200 | 26k | 40.3ms | 0.130ms |
| 500 | 66k | 240.0ms | 0.388ms |
| 1000 | 132k | 1190.0ms | 1.236ms |

Doubling the document from 500 to 1000 paragraphs multiplies the cost of
streaming it by 5.

**What a person experiences.** No single token is slow — 1.2ms at two pages is
invisible next to a frame — so this is not the stutter #5378 was. It is main-
thread work that grows with the response: 1.2s of parsing spread across a
two-page answer today, four times that at four pages, on a device that may also
be rendering it.

**Shape of a fix.** The settled prefix is immutable by construction, so all four
of these can work on the tail: keep the line count and fence state for the
settled prefix in `IncrementalState` and scan only from the boundary; keep the
link-definition signature per settled prefix and extract only from the tail; and
hand back the previously-returned array when the settled blocks are reused
unchanged. That is a change to what `IncrementalState` carries, not a repair —
worth its own PR and its own before/after on the table above.

The invariant to assert is the one [#5379](https://github.com/facebook/astryx/pull/5379)
established for the cache: an integer that stays flat across a 10x document, not
a millisecond budget. Here it would be the cost of one chunk relative to the
document length.